1. Field of The Invention
The present invention relates to a method of growing a single crystal of a rare-earth silicate, a method of machining a single crystal of a rare-earth silicate into a cylindrical form, a single crystal of a rare-earth silicate machined into a configuration having at least one plane e.g. mirror-finished plane) and a process for producing a single crystal of a rare-earth silicate having at least one plane. The single crystal is used in, for example, X-ray detectors as scintillators and other electronic equipments.
2. Description of Related Art
The single crystal of a rare-earth silicate such as gadolinium oxyorthosilicate is now widely being used in scintillators, phosphors and the like.
The gadolinium oxyorthosilicate single crystal belonging to monoclinic system can be produced from a melt of raw materials according to the Czochralski technique. That is, the single crystal can be produced by heating raw materials in a crucible to thereby obtain a melt of the raw materials, contacting a seed crystal with the melt and pulling the seed crystal to thereby grow a single crystal on the seed crystal. For use in scintillators and the like, the single crystal is generally machined into a cylindrical or rectangular-parallelepipedic shape. In practice, further, mirror-finish polishing is effected on at least one plane of the single crystal, a reflector is coated on or wound round the planes of the single crystal except one mirror-finished plane, and the reflector-free plane is fitted onto a photomultiplier prior to use.
However, the gadolinium oxyorthosilicate single crystal belonging to monoclinic system has the property of cleaving along the a-plane ((100) plane) and also the anisotropy that the coefficient of thermal expansion along the direction of the b-axis ([010] axis) is greater than those along other directions, so that not only is the single crystal likely to crack at the (100) and (010) planes during the cooling process for crystal growth but also new cracks are likely to occur during the machining thereof.
Methods for growing a crack-free single crystal of rare-earth silicate have been proposed, which include one aligning an axis of pulling so as to agree with the b-axis ([010] axis) of the single crystal or a vicinity thereof (a gradient of 0.degree. to 30.degree. from the b-axis) (Japanese Patent Application Laid-Open (KOKAI) No. 3-80183) and another aligning an axis of pulling so as to be parallel to the (100) plane and to have a gradient of 0.degree. to 25.degree. from the c-axis ([001] axis) (Japanese Patent Application Laid-Open (KOKAI) No. 4-175297).
However, these conventional methods have a problem that cracking still cannot be avoided during the cooling process of the growth of the single crystal of rare-earth silicate or during the machining of the single crystal.
Specifically, when the axis of pulling is aligned so as to agree with the b-axis ([010] axis) or a vicinity thereof, the seed crystal or grown crystal is cracked at a middle thereof during the cooling process of the growth, thereby causing the crystal to fall. Further, when a long cylindrical specimen is taken from the crystal grown in the above-mentioned direction, it is carried out along the length of the grown crystal (with the axis of the cylinder aligned so as to agree with the direction of the pulling axis). In taking the specimen, cracking is likely to occur during the cylinder grinding of the side face thereof, thereby disadvantageously lowering yield.
On the other hand, when the axis of pulling is aligned so as to agree with the [001] axis or a vicinity thereof, cleavage occurs during the grinding of a plane perpendicular to the direction of pulling in machining the specimen taken from the grown crystal. For avoiding the cracking of the specimen face, it is required that the specimen be taken with the center axis of the specimen held at a substantial gradient from the direction of the pulling axis. This disadvantageously lowers the specimen yield and complicates the operation.
As mentioned above, when the gadolinium oxyorthosilicate single crystal or the like belonging to monoclinic system is used in scintillators and the like, it generally has a cylindrical or rectangular-parallelepipedic shape. In practice, further, mirror-finish polishing is effected on at least one plane of the single crystal, a reflector is coated on or wound round the planes of the single crystal except one mirror-finished plane, and the reflector-free plane is fitted onto a photomultiplier prior to use.
The above-mentioned single crystal has strong cleaving properties and thus is likely to crack at the (100) plane. Hence, it is a relatively brittle crystal, thereby being likely to suffer from cracking during the cooling for crystal growth or the machining operation. Despite the above-mentioned properties, a cylinder grinding of the side face of the single crystal has been carried out with the axis of the cylinder aligned so as to agree with any arbitrary crystal orientation between the b-axis ([010] axis) and the c-axis ([001] axis) in the machining of the single crystal of rare-earth silicate such as gadolinium oxyorthosilicate into a cylindrical form.
This conventional method has a problem that cracks and peels are likely to occur at the side face during the cylinder grinding, thereby rendering a stable formation of smooth side surface unfeasible.
Further, when the gadolinium oxyorthosilicate single crystal or the like belonging to monoclinic system is used in scintillators and the like, generally, a cylindrical or rectangular-parallelepipedic single crystal is machined into a configuration having at least one plane prior to use.
The above-mentioned single crystal has strong cleaving properties and thus is likely to crack at the (100) plane. Hence, it is a relatively brittle crystal, thereby being likely to suffer from cracking during the cooling for crystal growth or the machining operation. In the machining into the configuration having at least one plane, generally, use is made of a technique machining the single crystal into a configuration having a plane parallel to a cleavage plane.
This conventional method has a problem that the cutting or grinding of the plane parallel to the cleavage plane is likely to cause cleavage plane peeling because the machined plane does not completely agree with the cleavage plane ((100) plane), thereby rendering obtaining a smooth machined surface unfeasible.